Philip Bagshaw, of the Department of Surgery, Christchurch School of Medicine, responded.

Mucus is normally secreted into the breathing passages of healthy people. It
acts there as a blanket to protect against damage from bacteria and other
particles which are breathed in during respiration. The protective mucus is
moved out of the breathing passages by special cells which have minute
hair-like projections called cilia. These beat rhythmically to move the
mucus on. Once it enters the throat the mucus is then swallowed.

The mucus layers have other protective roles. For example, it stops the
cells lining the air passage from drying out.

When we have a cold or other infection of the lungs or air passages, the
rate of production of the protective mucus is increased, and we all know of
the large amounts produced when we have a cold. We are usually unaware,
however, of the small amount which is always present when we are healthy.